Annealing for carbide decomposition

Thermal treatment of cast iron alloys for elimination of free carbides, e.g. of cementite.

In addition to an overview of the major annealing treatment methods for cast iron, Figure 1 also shows a schematic diagram of the annealing curve for this type of heat treatment. The annealing temperatures range between 850 and 950 °C at a heat-up speed of 50 to 100 K/h. The hold time depends on the wall thicknesses and is around 2 hours for 25 mm wall thickness plus 1 h per additional 25 mm. Cooling is performed in the air (furnace) at 40 to 60 K/h and depends on the requirements for structure and properties. For excellent tensile strength and wear resistant predominantly pearlitic structures must be achieved, which involves rapid cooling in air. If good machining properties are required, slow cooling rates (cooling in the furnace), particularly within the eutectic zone must be achieved (s. Heat treatment).

 

  • Fig. 1: Most important heat treatment methods for cast iron (schematic)
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